Viva libFM - Steffen Rendle on how he won the Grockit Challenge

Grockit competition winner, Steffen Rendle, shares his Factorization Machine technique. In his own words, "The combination of FMs and Bayesian learning was very handy as I didn't had to search for any regularization hyperparameters."

What was your background prior to entering this challenge?

I am an assistant professor in computer science at the University of Konstanz.

What made you decide to enter?

I wanted to study factorization machines on a competitive setting and get some empirical evidence that they work well. The Grockit challenge raised my interest because the dataset is of reasonable size (not too small) and has interesting variables.

What preprocessing and supervised learning methods did you use?

I used factorization machines (FM) which is a model class that I have developed over the last years. The model allows one to use feature engineering the same way as most standard machine learning tools (e.g. linear regression or SVMs) but it uses factorized variable interactions. For learning, I extended the Bayesian approach that was developed by Christoph Freudenthaler and me with classification capabilities.

I used a simple preprocessing that removes cases (student-question-combinations) that appear very often.

What was your most important insight into the data?

The most important predictors were the student, question and the time taken for answering a question. I also derived a variable that states how often a student has tried to answer a question before. Besides this I only used the subtrack name and the game type. All other variables didn't help -- at least not in my experiments.

Were you surprised by any of your insights?

Ensembles didn't work at all. I couldn't find any improvement by ensembling several models -- neither on my validation set, the public or private leaderboard. I didn't expect large improvements as I tried to have strong single models, however I was surprised to find no improvement at all.

Which tools did you use?

I used libFM which is an implementation of factorization machines. The software is written in C++ and can be obtained from my website http://www.libfm.org/

For preprocessing, I used Perl.

What have you taken away from this competition?

I think the format of machine learning challenges gives very valuable feedback to researchers. When conducting experiments for a research paper, one cannot compare to so many different approaches as you have in a Kaggle competition.